This probably is just an ambiguously romantic illustration of implied sex. Asu Tora's art is sometimes hard to read on top of being astonishingly beautiful, making it all the more interesting to look at!
However with Reimu's attitude and expression, I also like to see this as her being pinned against a wall by an unknown assailant and she's like:
"You sure you want to go down this road, with the Hakurei Shrine Maiden?" before whooping his ass.
